Name Answer Key
1) At standard temperature and pressure, bromine (Br2) is a
red liquid. Use the phase diagram for bromine is
(shown at the right) to answer the following questions.
Note: 1 atm = 101.3 kPa. Thus, 101.3 kPa is the
pressure for all “normal” mp and bp’s.
B
Normal Melting Pt
75 kPa
a) Label each region of the graph as solid, liquid, or
gas.
b) Label the triple point, normal melting point, and
normal boiling point on the graph and estimate their
values in the spaces below.
Normal Melting Point = –8°C
A
70 kPa
Normal Boiling Pt
WKS
Phase Diagrams
15°C
Normal Boiling Point = 59°C
Triple Point: P = 5 kPa
T = –9°C
Triple Pt
c) Explain the significance of the triple point?
The triple point is the pressure and temperature at which all 3 phases coexist in equilibrium.
d) When external pressure is increased, what happens to bromine’s melting point? Increase or decrease
e) Based on the slope of the melting-point curve in the phase diagram, would you characterize the
solid phase of bromine as more dense or less dense than the liquid phase of bromine? Explain
your reasoning.
Since the solid-liquid phase boundary has a positive slope, as the pressure is increased (points A
→ B), the density increases and the Br2 transitions from liquid to solid.
f)
g)
h)
i)
What is the boiling point of bromine when the external pressure is 75 kPa? 50°C
Bromine vapor at 15°C (condenses, sublimes) when the pressure is raised to 50 kPa.
Bromine liquid at 70 kPa (vaporizes, freezes) when the temperature is decreased to –15°C.
What is the vapor pressure of liquid bromine at 0°C? 10 kPa
2) a) What is sublimation?
The change directly from solid phase to gas phase.
b) Give two examples of common substances that sublime at ordinary temperatures.
I2 and CO2 were given in class. H2O sublimes as well, but only at low temperatures (snow on a
cold, sunny day, ice cubes in the freezer). Other examples found by searching the internet are
naphthalene, C10H8, and arsenic.
3) What is meant by the normal freezing point of a substance?
The temperature at which a substance changes from liquid to solid (or solid to liquid) at 1 atm
pressure.
4) Use the phase diagram of water (shown at right), to answer the following questions:
a) What phase change occurs when water goes from
100°C to −50°C at 0.3 kPa? deposition
b) What would happen to solid ice (originally at −5°C
and 1 atm), if the external pressure is greatly
increased?
It would melt.
c) By just looking at the phase diagram, how is one able
to tell that solid water is less dense than liquid water?
Because the solid-liquid boundary has a negative slope
(up & to the left)
d) Point “D” on the diagram is the critical point for water.
Thus, the critical temperature is 374°C for water.
What exists above this critical temperature?
A supercritical fluid, essentially a gas with a density about ½ that of a liquid.